RUSSIA’S APPARENT violation of the 1987 Intermediate-Range Nuclear Forces Treaty has moved a worrisome step forward. A ground-launched cruise missile that the United States has identified as a treaty violation is being deployed by Russia, according to a report in the New York Times. This threatens to upend an important treaty and poses a major challenge for the United States, especially since years of objections over the violation have been stonewalled by President Vladimir Putin. With a new U.S. administration taking office, it would make sense for President Trump to press Russia once more to adhere to the treaty, while holding out the possibility of military countermeasures if he does not.

The treaty was a centerpiece of the cooperation between President Ronald Reagan and Soviet leader Mikhail Gorbachev at the end of the Cold War, eliminating an entire class of deployed land-based missiles in Europe with a range between 300 and 3,400 miles, and their launchers; prohibiting flight-testing and production of new missiles in the future; and including new, intrusive verification measures.

Over the past decade, Russia stealthily developed a ground-launched cruise missile in apparent violation of the treaty, one of a number of asymmetric weapons programs developed by Mr. Putin to throw the West off balance. The new missile was first seen in a flight test in 2008; the Obama administration told Congress about it in late 2011, and State Department compliance reports formally called it a treaty violation in 2014, 2015 and 2016.

Russia has repeatedly refused to acknowledge a treaty violation. A special commission set up in the treaty to resolve disputes met late last year without result. Now, Mr. Trump should raise the issue directly with Mr. Putin and make it clear that the United States will not tolerate behavior that undermines the very foundation of arms-control treaties — that they are binding and verifiable. Mr. Trump has described himself as a good negotiator and as a Mr. Fix-it. Certainly, the INF treaty needs repair. The military countermeasures prepared by the Pentagon, such as deployment of new U.S. missiles or active defenses, might, over time, coerce the Kremlin to change tack. But it would be far preferable for Mr. Trump, who has yet to meet Mr. Putin, to attempt persuasion first, while being direct about the consequences of inaction.

According to an account by Reuters, Mr. Trump denounced another U.S.-Russian accord, the New START (Strategic Arms Reduction Treaty), in a Jan. 28 phone call with Mr. Putin, saying it favored Russia. In fact, the treaty has been a model of successful implementation, holds both nations to equal levels and ought to be extended when it expires in 2021. The real worries about strategic nuclear weapons are elsewhere, starting with setting priorities for the hugely expensive nuclear modernization cycle that the United States has embarked upon. There’s also the unratified nuclear Comprehensive Test Ban Treaty and the toothless and drifting Biological and Toxin Weapons Convention. Plenty of negotiating and fixing up awaits Mr. Trump, and none is getting easier by waiting.